1. Field of the Invention
The present invention relates to technology to prevent deviation of dots occurring with a recording apparatus, and more particularly to a technology to prevent deviation of dots occurring in a bidirectional recording operation performed by an ink jet recording apparatus.
2. Description of the Related Art
Hitherto, a recording apparatus for recording information on a recording medium, such as paper or an OHP sheet, has been suggested while being formed into a shape having a recording head mounted thereon, the recording head being adapted to any one of various recording methods such as a wire dot recording method, a thermal recording method, an ink jet recording method or the like.
A typical recording apparatus employs a serial printer. The serial printer records or prints (hereinafter called "records") data while moving a carriage having a recording (printing) head mounted thereon with respect to the recording medium. An example of the conventional structure of the foregoing type is shown in FIG. 16. FIG. 16 is a perspective view which illustrates a conventional recording apparatus.
As shown in FIG. 16, a recording head 1002 is fastened to a carriage 1001. While allowing the carriage 1001 to reciprocate in parallel to a platen 1003 (in directions designated by arrows a and b shown in FIG. 16), a recording medium 1004 is conveyed for one line (in a direction designated by an arrow c shown in FIG. 16) so that data items on the ensuing lines are recorded by the recording head 1002. In this case, ink is discharged from the recording head 1002 only when the head moves in the direction designated by the arrow b shown in FIG. 16.
As for the recording medium, it is an important fact that data can be recorded onto both a thick sheet, such as an OHP film, a post card, and an envelope, as well as the usual copying paper. From the foregoing viewpoints, the structure of the recording apparatus has been arranged in such a manner that the distance from the recording head to the recording medium can be varied.
As described above, the conventional recording apparatus must simplify the elements constituting the recording apparatus and mechanisms that respectively connect the elements to one another, reduce the size of the apparatus, and increase the recording speed.
In contrast to the foregoing recording method in which the recording operation is performed in only the single direction, bidirectional recording in which the recording operation is performed when the carriage is returned as well offers an advantage in terms of increasing recording speed. Therefore, the bidirectional recording must be performed to raise the operational speed of the recording apparatus. However, when that is done, the direction in which the carriage employed in the ink jet recording method for recording data by discharging ink is moved in one direction in the forward recording operation and in the other direction for the rearward recording operation. Therefore, it has been known that the recording operation involves a deviation in principle. Further, the quantity of the deviation varies depending upon the distance from the recording head to the recording medium.
FIG. 17 is a schematic view which illustrates a state where ink is discharged from a recording head of an ink jet recording apparatus to a recording medium.
In FIG. 17, Vd is ink droplet discharge speed and Vcr is the moving speed of the carriage, the direction of the movement of the carriage being reversed between the forward movement and the rearward movement. Symbol L denotes the distance from the recording head to the recording medium.
The ink droplet discharged at a specific time shown in FIG. 17 has a speed component V obtained by synthesizing Vd and Vcr at the time at which the discharge is performed. The ink droplet reaches the recording medium (L/Vd) seconds after the discharge. At this time, the ink droplet reaches a position shifted in direction x by a quantity expressed by Vcr.times.(L/Vd). Although the position at which the ink droplet is received by the recording medium is deviated whenever one line is recorded, the deviation cannot be recognized in the final product of the one-directional recording process because the shifts are performed by the same amount in the same direction for each line.
On the contrary, in bidirectional recording the recording operation is performed during both forward and rearward movement of the carriage, the direction of the carriage movement at a moving speed of Vcr being reversed between the forward and the rearward movements. Therefore, the position at which the ink droplet is received deviates for each line by a quantity expressed by 2.times.Vcr.times.(L/Vd). There arises a problem to be solved so as to improve the quantity of the recorded product.
A recording apparatus comprising a plurality of recording means disposed to be adaptable to different-color recording materials, such as ink, and mounted on a carriage thereof and arranged for moving the recording means in a predetermined direction with respect to a recording medium, such as recording paper, an OHP film or a cloth sheet to record data by superposing colors suffers from a problem in that slight deviations take place in the superposition of the colored dots in the moving direction. In particular, the desired multi-color images cannot be obtained due to the undesirable effect on the superposition of the colors. In order to prevent color deviations from occurring in the moving direction, a structure has been employed which is arranged in such a manner that a test pattern is recorded at the time of the assembly or the adjustment of the recording apparatus. Further, the recorded dot superposition of the colors is used to adjust precisely the position of recording means with respect to a standard recording means to fix the recording means.
As another method of preventing color deviations in the head moving direction, a structure has been employed in which the recording means is fixed to a predetermined position, and the timing at which the other recording means perform recording is adjusted with respect to the recording timing of the standard recording means in accordance with the degree of the color deviation.
However, even if the dot deviations occurring among the recording means can be completely adjusted at the time of the assembly or the adjustment, the relative movement speed is later changed due to expansion or contraction of the means, for example, the carriage, that causes the foregoing relative movement should the environment of the recording apparatus be changed, and more particularly if the temperature is changed. In this case the dots deviate because of changes in the relative movement speed.
The foregoing problem will now be described with reference to FIG. 18. The apparatus shown in FIG. 18 comprises a carriage 1102 on which ink jet heads 1101A, 1101B, 1101C and 1101D corresponding to four colors (for example, yellow, magenta, cyan and black) ink are mounted. The carriage 1102 is caused to reciprocate along a guide rail 1107 in the lateral direction by the operating force transmitted from a motor 1103 for moving the carriage 1102 so that data is recorded on recording paper sheet 1108. The operating force generated by the motor 1103 is transmitted to the carriage 1102 by way of a wire 1109 arranged between a motor pulley 1104 and an idler pulley 1105, the wire 1109 being fixed at a fixing portion 1102a of the carriage 1102.
Assuming that the ink jet heads 1101A to 1101D are equally mounted on the carriage 1102 at pitch L and data is recorded when the carriage 1102 is moved at speed V to the right portion of FIG. 18, data can be recorded on the same point by the standard ink jet head 1101A and the ink jet head 1101B when the ink jet head 1101B performs the recording operation .DELTA.T.sub.2 =L/V after the moment at which the standard ink jet head 1101A has performed the recording operation. Further, the residual ink jet heads 1101C and 1101D must perform the recording operations .DELTA.T.sub.2 =2L/V and .DELTA.T.sub.3 =3L/V after the ink jet head 1101A has performed the recording operation. If V=300 mm/sec and L=30 mm, the following requirements arise: .DELTA.T.sub.1 =0.1 sec, .DELTA.T.sub.2 =0.2 sec and .DELTA.T.sub.3 =0.3 sec. If the rotational speed of the motor 1103 is 300 rpm, the diameter of the motor pulley 1104 will be about 18 mm (if the diameter of the wire is 1 mm). However, somewhat deviations of V and L from the designed values due to manufacturing errors or the like raise the necessity of further allowing electrical adjustment of the recording timing adjustment quantity .DELTA.T.sub.1 to .DELTA.T.sub.3 to be performed at the time of the assembly or the adjustment of the recording apparatus to correct the recorded dot.
Even if the deviation of the recorded dots to be superposed is corrected at the time of assembly or adjustment of the apparatus, differences in the environment in which the apparatus is used will cause motor pulley 104 to expand or contract. In this case, the recorded dots desired to be superposed are sometimes deviated. An example of that case will now be described in which a recording apparatus having the foregoing dimensions has been adjusted at 28.degree. C. and it is used at 13.degree. C., that is a temperature difference of 15.degree. C. takes place. If the motor pulley is made of polyacetal having a linear expansion coefficient of about 8 to 11.times.10.sup.-5 1/.degree. C. the diameter of the pulley changes by 27 .mu.m for the case where it is assumed that the expansion coefficient is 10.times.10.sup.-5. If the recording apparatus is operated in the foregoing state, the speed V is 300.times.(18-0.027)/18=299.55 (mm/sec).
Considering the recording state, the head 1101B forms a dot at a position in front of the dot formed by the ink jet head 1101A by 30-(299.55.times.0.1)=0.045 (mm). Similarly, the ink jet heads 1101C and 1101D form dots at positions in front of the same by 0.090 mm and 0.135 mm, respectively. As a result, the recorded dots are deviated as described above. Since a 360 DPI (dots/inch) or a 400 DPI head, the image density of which is raised, has a narrow dot pitch of 0.0706 mm or 0.0635 mm, the foregoing deviation is larger than the dot pitch and so there arises a critical problem because the color of the recorded product can be changed considerably. Even if a timing belt is used as the transmission means in place of the wire, a similar problem arises although the influence upon the speed can be eliminated satisfactorily. What is worse, another problem arises when another timing belt is used in place of a broken timing belt in that scattering of the length of the timing belt arises the undesirable difference in the drive speed. Therefore, the deviation of the recorded dot must be adjusted again, requiring the complicated adjustment operation to be performed.